Response to Raising FiO2
True shunt (V/Q = 0)
- Shunted blood bypasses ventilated alveoli entirely.
- Its PO2 stays near mixed-venous level (~40 mmHg).
- Raising FiO2 raises PO2 only in the normal regions.
- The shunted blood dilutes the arterial mixture, so PaO2 rises little.
- Result: hypoxemia is refractory to supplemental oxygen.
Dead space (V/Q = infinity)
- Ventilation is wasted on unperfused alveoli.
- The remaining perfused alveoli receive the higher alveolar PO2.
- CO2 elimination depends on the remaining perfused alveoli increasing their ventilation.
- Supplemental oxygen improves PaO2 because the problem is wasted ventilation, not blocked diffusion.
- Result: hypoxemia responds to supplemental oxygen.
The reason shunt is refractory is anatomical, not chemical. Oxygen therapy can only raise the PO2 of alveoli that are actually ventilated. Blood that never reaches a ventilated alveolus cannot be oxygenated by any increase in inspired oxygen. The only way to correct a true shunt is to restore ventilation to the affected alveoli or to reduce the fraction of cardiac output passing through them.
Real lungs rarely contain a pure shunt or pure dead space. Most hypoxemia reflects a mixture of low-V/Q regions that behave like partial shunts and high-V/Q regions that waste some ventilation. The arterial blood gas is the flow-weighted result of all these contributions, which is why the response to oxygen therapy is usually partial rather than all-or-nothing.